This invention relates to the construction of a solid board, such as a door, and more specifically to a fire rated door formed of a solid core and wooden strips attached around its edges.
Typical present solid door construction include three basic components: a core, wood edges attached around the core (the vertical edges being referred to as stiles and the horizontal edges as rails), and thin facing material covering both sides of the door for its appearance. A principal factor taken into account in choosing the materials for the core and the wooden edges, and also for determining the thickness of the wooden edges, is the fire rating desired for the door. Building codes require that doors to be installed in certain building positions need to have a particular fire rating that is measured in time, such as a 20 minute door, or a 45 minute, one-hour or one and one-half hour door. Doors are given a fire rating in accordance with a standard test specification. A leading test is ASTM E-152 (1976). Others are UL 10(b) (1974), NFPA 252 (1972) and UBC 43-2 (1973), all similar to the ASTM test. In conducting such tests, doors are mounted in an opening of a fireproof wall and then exposed on one side to a predetermined time-temperature rise function. The time that a door can withstand the heat before it is penetrated by burning determines its fire rating.
Fire door core materials commonly used at the present time include untreated wood or particle board for doors of a low fire rating, such as 20 minutes, or a particle board treated with a fire retardant, or a mineral core for doors of the higher fire rating, such as 45 minutes or more. A one-hour rated mineral door core is presently commercially available from the Gypsum Division of the Georgia-Pacific Corporation.
Wood stiles and rails (edges) are attached to the core edges by an appropriate adhesive, usually by the door manufacturer, in order to provide edges that will hold wood screws used by the purchaser of the door to mount normal hardware thereon, such as hinges and door latching mechanisms. Presently available doors utilize solid wood stiles and rails that have been treated with a fire retardant, often in a salt form. Hemlock and maple are popularly utilized wood species for door stiles and rails. As the desired fire rating of the door goes up to 45 minutes or more, the stiles and rails must be made very narrow. The reason for this is that such fire retardant treated solid wood material cannot withstand the heat of a standard fire test for such long periods of time without being penetrated by burning. Therefore, the stiles and top rail are made to be as narrow as the door stop on a frame on which the door is to be mounted during the fire test. The standard fire tests identified above test for fire penetration during the test period of only the door portion between door stops. That is, penetration of the door edges behind the door stops does not disqualify the door; it passes the test anyway because no penetration is visible. The door core is made to overlap the door stops. Typical dimensions for such a long fire rated door are stiles of 3/4 inch wide, a top rail of 5/8 inch wide and a bottom rail of 1-1/4 inch wide.
Such a narrow stile, necessitated by the desired fire rating of the door, has low resistance to splitting along its grain and a low ability to hold wood screws. The core material provides no screw holding power. Commercially available doors of all types are listed in manufacturers' product catalogs accumulated in "Sweet's Catalog File: Architectural Products for General Building", Volume 5, Section 8.3 (1977), published by the McGraw-Hill Information Systems Company and widely used by architects.
It is a principal object of the present invention to provide an improved solid board, such as a door, having a high fire rating but, with more substantial wooden edges for better resisting wood screw withdrawal and splitting than present doors of an equivalent fire rating.